C–H stretching vibrations of benzene and toluene in their S1 states observed by double resonance vibrational spectroscopy in supersonic jets

Abstract

An investigation of CH stretching vibrations in benzene and toluene in their S₁ states has been carried out using UV–IR and stimulated Raman–UV double resonance spectroscopic methods. In benzene two CH stretching vibrations, namely ν′₂₀ and ν′₂, were observed, and in the case of toluene both aromatic CH and methyl CH stretching vibrations were observed. The ν′₂₀ of benzene is strongly coupled with ν′₈ + ν′₁₄via anharmonic resonance. The aromatic CH stretching vibrations in toluene also exhibit strong anharmonic resonance, leading to the appearance of a large number of bands in the 3000–3100 cm⁻¹ region. The observed frequencies of CH stretching vibrations in the S₁ state in both benzene and toluene are higher than the corresponding values in the S₀ state. On the other hand, methyl CH stretching vibrations in the S₁ state of toluene occur at a lower frequency than those in the S₀ state.